Equipment of this type usually comprises a portion (stator) that is fixed to the chassis of the vehicle and that includes inductor windings, and a moving portion (rotor) including an armature and coupled to a rotary element of the vehicle, generally its transmission shaft.
The term "inductor winding" or more simply "winding" is used herein to cover both an inductor winding proper and a group of such windings that are permanently interconnected in series and/or parallel. Each winding as defined in this way produces a magnetic field when powered by the vehicle battery.
The armature is an element of ferromagnetic material which, when moving past excited windings, has electrical currents known as "eddy" currents induced therein. Because of the resistivity of the armature, these eddy currents cause energy to be dissipated and this results in the rotor, and thus the vehicle, being slowed down. The energy is dissipated in the form of heat, and the rotor is commonly given a finned configuration suitable for disposing of said heat.
The driver of the vehicle can actuate a multiple-position control lever to obtain a braking effect on the vehicle with a torque that varies depending on the position selected for the lever. This variability is obtained by a set of relays each serving to excite one of the windings with the number of relays in the closed-circuit position depending on the position of the lever. In a typical embodiment, there are four inductor windings and the lever has five positions corresponding respectively to 0, 1, 2, 3, and 4 of the relays being closed, with corresponding proportional braking torques being obtained.
In any given position, it is known that the braking torque tends to decrease as the temperature of the armature increases, because of variations in its resistivity and in its magnetic permeability. Beyond a certain temperature threshold, it is advantageous to limit excitation of the windings so as to avoid loss of available torque which would give rise to excessive heating. This limitation also makes it possible to achieve better management of the electrical energy resources of the vehicle.
FR-A-2 505 577 describes one way of achieving such excitation limiting. A temperature-sensitive contact is housed in a projection on a pole piece in a face of the stator adjacent to the rotor, and it is used to detect when the temperature threshold is exceeded. The contact is sensitive to the heat radiated by the armature and it is thus suitable for providing the desired temperature indication.
That type of temperature sensor is also provided by certain manufacturers who make rotors having portions that are unsuitable for withstanding large increases in temperature. This applies to a rotor having an armature made of steel which is supported on aluminum, since aluminum does not withstand temperatures greater than 300.degree. C.
Since the rotor moves, the sensor cannot be in direct thermal contact with the armature, without using a structure that is excessively complicated. The sensor must therefore be mounted on the stator to receive the radiation given off by the armature, and this gives rise to three main problems that have not yet been solved:
random dirtying of the sensor disturbs its measuring ability by impeding its reception of radiation;
it is difficult to make the sensor substantially independent of the temperature of the stator on which it is mounted; and
it is difficult to make a sensor that is relatively insensitive to heat conveyed from the rotor by air flow; unfortunately, air flow is never fully under control, particularly because of the draft produced by the fins on the rotor.
In addition, it can be difficult to install a sensor in certain models of brake.